Diffraction gratings for beam redirection

ABSTRACT

A diffraction grating with independently controlled diffraction angles for optical beams at different wavelengths may be used to redirect and couple light to a waveguide in an efficient, space-saving manner. The diffraction grating can include a layer with optical permittivity and associated index contrast of the grating grooves at different grating periods dependent on wavelength.

REFERENCE TO A RELATED APPLICATION

The present application claims priority from U.S. ProvisionalApplication No. 62/667,393 entitled “Diffraction Gratings for BeamRedirection” filed on May 4, 2018 and incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present disclosure relates to optical components, and in particularto diffraction gratings suitable for redirecting optical beams, andmodules and systems using such diffraction gratings.

BACKGROUND

Diffraction gratings are optical devices for separating light atdifferent wavelengths by using the phenomenon of optical diffraction ona periodic or quasi-periodic grating structure. The angle of diffractionof light depends on the ratio of the wavelength of light to the periodof the periodic or quasi-periodic grating structure, causing thediffraction grating to disperse an impinging multi-wavelength opticalbeam into a fan of sub-beams at different wavelengths. The wavelengthseparating property of diffraction gratings, in combination with highachievable diffraction efficiency and a relative ease of manufacture,resulted in their widespread use in spectrographs, lasers,wavelength-selective optical switches, tunable filters, and otherdevices.

Diffraction gratings may be used as anamorphic optical elements forspace-efficient redirection of optical beams. The use of diffractiongratings for beam redirection is attractive because a diffractiongrating can be constructed to efficiently redirect the beam at obliqueangles with respect to the plane of the diffraction grating, savingspace by suitable placement of the diffraction gratings parallel toavailable substrates, and compressing circular beams into beams havingan elliptical cross section for propagation in thin and narrow waveguidestructures. However, the beam redirecting and coupling of multi-coloredbeams is hindered by dependence of the angle of diffraction onwavelength, resulting in spatial separation of wavelength sub-beams of amulti-wavelength optical beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A is a side cross-sectional view of an optics block of a near-eyedisplay using diffraction gratings for pupil replication according to anembodiment of the present disclosure;

FIG. 1B is a side cross-sectional view of an optics block of a near-eyedisplay including three grating structures and three waveguides forthree color channels, according to an embodiment of the presentdisclosure;

FIG. 2A is a schematic diagram showing a wavelength dependence ofamplitudes of spatial variation of optical permittivity corresponding togrooves of two diffraction grating structures in accordance with thepresent disclosure;

FIG. 2B is a schematic diagram showing a wavelength dependence ofamplitudes of spatial variation of optical permittivity corresponding togrooves of three diffraction grating structures in accordance with thepresent disclosure;

FIG. 2C is a spectral plot of refractive index contrast of the threegrating structures of FIG. 1B, according to one embodiment;

FIG. 3A is a side cross-sectional view of an optics block for a near-eyedisplay including three adjacent grating structures and one commontransparent slab, according to an embodiment of the disclosure;

FIG. 3B is a magnified view of the adjacent grating structures of FIG.3A;

FIG. 4 is a three-dimensional plot of optical permittivity of adiffraction grating material as a function of wavelength andx-coordinate, showing a spatially varying optical permittivity havingamplitude and period of the spatial variation dependent on wavelength;

FIG. 5 is a side cross-sectional view of an embodiment of thediffraction grating of FIG. 4, in which the diffraction angles of threecolor channels are substantially equal to each other;

FIG. 6A is a side cross-sectional view of a diffraction gratingcomprising two distinct layers with embedded nanoparticles havingdistinct surface plasmon resonances;

FIG. 6B is a side cross-sectional view of a diffraction gratingcomprising a single layer of material with embedded nanoparticles havingdistinct surface plasmon resonances;

FIG. 6C is a top view of a diffraction grating comprising a layer ofmaterial with nanoparticles adhered to a top surface of the layer, thenanoparticles having distinct surface plasmon resonances;

FIG. 6D is a top view of a diffraction grating comprisinglithographically formed sub-arrays of nanoparticles having distinctsurface plasmon resonances, the sub-arrays forming grooves or stripes ofdiffraction grating structures with different grating periods;

FIGS. 7A to 7F are side cross-sectional views of the diffraction gratingof FIG. 6C at different stages of manufacture;

FIGS. 8A to 8D are views of different nanoparticles for use indiffraction gratings of FIGS. 6A to 6D;

FIG. 9 is a side cross-sectional view of a diffraction gratingcomprising a hyperbolic metamaterial;

FIG. 10A is a permittivity spectrum of silver (Ag) and silicon (Si)materials;

FIG. 10B is a permittivity spectrum of a hyperbolic metamaterialcomprising a stack of thin Ag and Si layers;

FIG. 11 is a spectral plot of refractive index contrast of thehyperbolic metamaterial of FIG. 10B;

FIG. 12 is a side cross-sectional view of a diffraction gratingcomprising two hyperbolic metamaterial sub-gratings;

FIG. 13 is a spectral plot of refractive index contrast of thediffraction grating of FIG. 12;

FIG. 14A is a side cross-sectional view of an optical waveguide fornear-eye display, including a plurality of grating structures on top ofone another;

FIG. 14B is a side cross-sectional view of an optical waveguide fornear-eye display, including a diffracting grating having a spatiallyvarying optical permittivity dependent on wavelength;

FIG. 15 is a side cross-sectional view of a near-eye display of thepresent disclosure;

FIG. 16A is an isometric view of an eyeglasses form factor near-eyeAR/VR display incorporating an optics block with diffraction gratings ofthe present disclosure;

FIG. 16B is a side cross-sectional view of the display of FIG. 16A; and

FIG. 17 is an isometric view of a head-mounted display (HMD)incorporating an optics block with diffraction gratings of the presentdisclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated.

The dependence of an output beam angle of a diffraction grating onwavelength occurs exactly due to the property of diffraction gratingsthey are typically used for, i.e. the wavelength separation anddispersion property. When a multi-color beam, e.g. a white light beam,is diffracted, it is spread into individual wavelength components, eachhaving its own direction of propagation, which may be undesirable inmulti-wavelength beam steering and coupling applications.

In accordance with the disclosure, a diffraction grating may beconstructed such that diffraction angles of different color componentscan be individually controlled. For example, sub-beams at wavelengthscorresponding to red, green, and blue color channels of a visual displaycan be made to diffract at a same angle, enabling efficient guiding of awhite-light beam through a compact optical system substantially withoutdispersing the white-light beam into individual color sub-beams.

In accordance with the present disclosure, there is provided adiffraction grating comprising first and second grating structures. Thefirst grating structure has a first spatial variation of opticalpermittivity. The first spatial variation has a first grating pitch anda wavelength-dependent first amplitude, such that at a first wavelength,the first amplitude is above a first threshold and at a secondwavelength, the first amplitude is below a second threshold. The secondgrating structure has a second spatial variation of opticalpermittivity. The second spatial variation has a second grating pitchand a wavelength-dependent second amplitude, such that at the firstwavelength, the second amplitude is below the second threshold and atthe second wavelength, the second amplitude is above the firstthreshold. The second threshold is lower than the first threshold, e.g.below 10% of the first threshold, below 1% of the first threshold, oreven close to zero.

In accordance with the present disclosure, there is further provided anoptical waveguide for a near-eye display. The optical waveguide includesa slab of transparent material and the first and second gratingstructures described above. The first grating structure may be supportedby the slab, and the second grating structure may be supported by thefirst grating structure.

The optical permittivity of the first grating structure can include aspatially varying refractive index having a wavelength-dependent firstrefractive index contrast, and the optical permittivity of the secondgrating structure can include a spatially varying refractive indexhaving a wavelength-dependent second refractive index contrast. In otherwords, the grating structures of the diffraction grating and/or theoptical waveguide may be purely refractive, each being characterized bya wavelength-dependent refractive index contrast. The first refractiveindex contrast may be above the first threshold at the first wavelengthand below the second threshold at the second wavelength. The secondrefractive index contrast may be below the second threshold at the firstwavelength and above the first threshold at the second wavelength. Insome embodiments, the first refractive index contrast can besubstantially zero at the second wavelength, and the second refractiveindex contrast can be substantially zero at the first wavelength.

A third grating structure may be provided for the above optical devices.The third grating structure may have a third spatial variation ofoptical permittivity. The third spatial variation may have a thirdgrating pitch and a wavelength-dependent third amplitude, such that atthe first and second wavelengths, the third amplitude is below thesecond threshold and at a third wavelength, the third amplitude is abovethe first threshold. In this embodiment, at the third wavelength, thefirst amplitude and the second amplitude are both below the secondthreshold. Each one of the first, second, and third wavelengths maycorrespond to a color channel of an electronic display. Ratios of thefirst grating pitch to the first wavelength; the second grating pitch tothe second wavelength; and the third grating pitch to the thirdwavelength may be selected to be equal to each other, such that inoperation, components of an optical beam at the first, second, and thirdwavelengths emitted by the electronic display and impinging on thediffraction grating are diffracted at a substantially same diffractionangle. The first and second grating structures may be disposed adjacentone another, e.g. on top of one another.

In some embodiments, the first grating structure may include a pluralityof first nanoparticles at a first spatially varying density ofnanoparticles, the first nanoparticles having a surface plasmonresonance at the first wavelength. The second grating structure mayinclude a plurality of second nanoparticles at a second spatiallyvarying density of nanoparticles, the second nanoparticles having asurface plasmon resonance at the second wavelength. The firstnanoparticles may include metal spheres of a first diameter, and thesecond nanoparticles may include metal spheres of a second diameter.Metal-semiconductor or metal-dielectric core shells, or both, of same ordifferent diameters and diameter ratios, may also be used for the firstand second nanoparticles. The third grating structure may include aplurality of third nanoparticles at a third spatially varying density ofnanoparticles, the third nanoparticles having a surface plasmonresonance at a third wavelength. Each one of the first, second, andthird wavelengths may correspond to a color channel of an electronicdisplay.

In some embodiments, the plurality of first nanoparticles comprises anarray of first stripes of nanoparticles at the first grating pitch, theplurality of second nanoparticles comprises an array of second stripesof nanoparticles at the second grating pitch, and the plurality of thirdnanoparticles comprises an array of third stripes of nanoparticles atthe third grating pitch. Ratios of the first grating pitch to the firstwavelength; the second grating pitch to the second wavelength; and thethird grating pitch to the third wavelength may be selected to be equalto each other to make sure that components of an optical beam at thefirst, second, and third wavelengths emitted by the electronic displayand impinging on the diffraction grating are diffracted at asubstantially same diffraction angle. Furthermore, each stripe caninclude a sub-array of nanoparticles.

In some embodiments, the first grating structure includes a firsthyperbolic metamaterial comprising an array of first stripes at thefirst grating pitch and having a zero refractive index contrast at thesecond wavelength, and the second grating structure comprises a secondhyperbolic metamaterial comprising an array of second stripes at thesecond grating pitch and having a zero refractive index contrast at thefirst wavelength. Each first stripe may include a stack of alternatingmetal and semiconductor layers having a first set of thicknesses toprovide a zero refractive index contrast at the second wavelength, andeach second stripe may include a stack of alternating metal andsemiconductor layers having a second set of thicknesses to provide azero refractive index contrast at the first wavelength. A stack ofalternating metal and dielectric layers may also be used for any of thegrating structures. A ratio of the first to second grating pitches maybe selected to be equal to a ratio of the first to second wavelengths,such that in operation, components of an optical beam at the first andsecond wavelengths impinging on the diffraction grating are diffractedat a substantially same diffraction angle.

In accordance with the present disclosure, there is further provided adiffraction grating comprising a layer of material with a spatiallyvarying optical permittivity dependent on wavelength, such that at afirst wavelength, the spatially varying optical permittivity comprises aplurality of peaks and valleys at a first pitch, and at a second,different wavelength, the spatially varying optical permittivitycomprises a plurality of peaks and valleys at a second, different pitch.

In accordance with the present disclosure, there is further provided anoptical waveguide for a near-eye display. The optical waveguide mayinclude a slab of transparent material and a diffraction gratingsupported by the slab. The diffraction grating may include a layer ofmaterial having a spatially varying optical permittivity dependent onwavelength, such that at a first wavelength, the spatially varyingoptical permittivity comprises a plurality of peaks and valleys at afirst pitch, and at a second, different wavelength, the spatiallyvarying optical permittivity comprises a plurality of peaks and valleysat a second, different pitch.

The spatially varying optical permittivity may include a spatiallyvarying refractive index. A ratio of the first to second pitches may beselected to be equal to a ratio of the first to second wavelengths, suchthat in operation, components of an optical beam at the first and secondwavelengths impinging on the diffraction grating are diffracted at asubstantially same diffraction angle.

The material of the diffraction grating may include a plurality of firstnanoparticles at a first spatially varying density, the firstnanoparticles having a surface plasmon resonance at the firstwavelength; and a plurality of second nanoparticles at a secondspatially varying density, the second nanoparticles having a surfaceplasmon resonance at the second wavelength.

The material may also include a hyperbolic metamaterial including anarray of first stripes at the first pitch and having a zero refractiveindex contrast at the second wavelength, and an array of second stripesat the second pitch and having a zero refractive index contrast at thefirst wavelength. Each first stripe may include a stack of alternatingmetal and semiconductor or dielectric layers having a first set ofthicknesses to provide a zero refractive index contrast at the secondwavelength. Each second stripe may include a stack of alternating metaland semiconductor or dielectric layers having a second set ofthicknesses to provide a zero refractive index contrast at the firstwavelength. A ratio of the first to second pitches may be selected to beequal to a ratio of the first to second wavelengths; in thisconfiguration, components of an optical beam at the first and secondwavelengths impinging on the diffraction grating are diffracted at asubstantially same diffraction angle.

In accordance with the present disclosure, there is further provided anear-eye display comprising an electronic display for providing imagelight comprising first and second color channels, and an opticalwaveguide configured to receive and guide therein the image light fromthe electronic display. The optical waveguide may be any of the opticalwaveguides described above. In one embodiment, the optical waveguideincludes a slab of transparent material. A first diffraction grating issupported by the slab and is configured for receiving the image lightfrom the electronic display for in-coupling to the slab. A seconddiffraction grating is supported by the slab and is configured forreceiving the image light from the first diffraction grating through theslab for out-coupling the light from the slab. At least one of the firstand second diffraction gratings comprises a layer of material having aspatially varying optical permittivity dependent on wavelength, suchthat at a first wavelength of the first color channel, the spatiallyvarying optical permittivity comprises a plurality of peaks and valleysat a first pitch; and at a second wavelength of the second colorchannel, the spatially varying optical permittivity comprises aplurality of peaks and valleys at a second, different pitch. A ratio ofthe first to second pitches may be selected to be equal to a ratio ofthe first to second wavelengths, to make sure that the first and secondcolor channels of image light at the first and second wavelengthsimpinging on the first diffraction grating are diffracted at asubstantially same angle for joint propagation in the slab, and thefirst and second color channels of image light at the first and secondwavelengths impinging on the second diffraction grating are diffractedat a substantially same angle for joint propagation towards an eye of auser.

Referring now to FIG. 1A, an optics block 100A of a near-eye display ofthe present disclosure includes a projector 172 and a pupil-replicatingwaveguide 174. The projector 172 includes an electronic display 176 andcollimating optics represented by a lens 178. The pupil-replicatingwaveguide 174 includes a plano-parallel slab 175 of a transparentmaterial, an in-coupling diffraction grating 181 supported by the slab175, and an out-coupling diffraction grating 182 supported by the slab175. The in-coupling 181 and out-coupling 182 diffraction gratings areshown in FIG. 1A with thick dashed lines. In operation, an image isdisplayed by the electronic display 176. The lens 178 can be disposedone focal length away from the electronic display 176. The lens 178transforms a diverging light cone emitted by a center pixel 184 of theelectronic display 176 into a collimated or nearly-collimated on-axisoptical beam 185 shown with solid arrows. The lens 178 also transforms adiverging light cone emitted by a side pixel 186 of the electronicdisplay 176 into a collimated off-axis optical beam 187 shown withdashed arrows. Thus, a coordinate of a light-emitting pixel of theelectronic display 176 is transformed by the lens 178 into an angle of acorresponding collimated or nearly-collimated optical beam. In otherwords, a combined optical beam 188 carries the image displayed by theelectronic display 176 in angular domain.

The function of the pupil-replicating waveguide 174 will now beexplained. The in-coupling diffraction grating 181 receives the combinedoptical beam 188 and diffracts the combined optical beam 188 forpropagation in the slab 175 in a zigzag pattern as shown. The directionof the zigzag propagation of the combined optical beam 188 in the slab175 is from left to right in FIG. 1A. The out-coupling diffractiongrating 182 receives the combined optical beam 188 from theplano-parallel slab 175 and outputs a portion of that beam towards auser's eye 136. A portion of the remaining light is reflected by theout-coupling diffraction grating 182 to be output at a downstreamlocation. The combined optical beam 188 is directed towards an uppersurface 175A of the slab 175, is reflected from the upper surface 175A,e.g. totally internally reflected, and impinges on the out-couplingdiffraction grating 182 again at a different location, which is offsetto the right in FIG. 1A. Then the process of reflection and diffractionrepeats, effectively expanding the combined optical beam 188 over anarea of an eyebox 190, while preserving the angular image information asshown by downward-looking arrows 189. In other words, this multiplereflection-diffraction process replicates the output pupil of the opticsblock 100A. This makes the eyebox 190 larger than the user's eye 136,allowing a degree of movement and rotation of the user's eye 136, andalso accommodating for a different disposition of the near-eye displaywith respect to eyes of different users, as well for a differentinter-pupil distance (IPD) of different users.

Cornea and lens 135 of the user's eye 136 focus the light onto a retina137 of the user's eye 136, transforming the beam angle back into a beamcoordinate on the retina 137, thereby forming an image 196 of the sidepixel 186 of the electronic display 176 at a different location of theretina 137 than an image 194 of the center pixel 184, and essentiallytransferring the image displayed by the electronic display 176 onto theretina 137 of the user's eye 136.

The optics block 100A is compact and has a small number of opticalelements, making it lightweight and suitable for a near-eye displayhaving a form factor of a pair of eyeglasses. However, it can only workin a limited wavelength band, and/or in a limited angular viewing rangeor gaze range. This happens because in a regular diffraction grating,the diffraction angle depends on the wavelength of impinging light, andfor wavelengths outside of a certain wavelength band, the diffractionangles become too small or too large for the pupil-replicating waveguide174 to work properly. The present disclosure seeks to expand the viewingangle, the wavelength band, or both, of a display system by providing adiffraction grating, in which the diffraction angles can be individuallycontrolled for different color channels and/or different wavelengths.This can expand the available design space, allowing one to reduce thedisplay size, weight, and/or to increase the viewing angle and colorrange, fidelity, and uniformity. It is to be understood however that thedisclosure is not limited to display systems only; the diffractiongratings described herein may be used in a broad variety of opticaldevices requiring optical beam redirection and routing. Thepupil-replicating waveguide 174 is just a non-limiting, illustrativeexample of an optical module or system where the diffraction gratings ofthe present disclosure can be advantageously used.

Turning to FIG. 1B, an optics block 100B has not one but three stackedpupil-replicating waveguides, which are similar to the pupil-replicatingwaveguide 174 of FIG. 1A. The optics block 100B includes firstin-coupling 101 and out-coupling 111 grating structures supported by afirst transparent slab 121, second in-coupling 102 and out-coupling 112grating structures supported by a second transparent slab 122, and thirdin-coupling 103 and out-coupling 113 grating structures supported by athird transparent slab 123. The slabs 121, 122, and 123 are arranged ina stack configuration as shown. A projector 132, similar to theprojector 172 of FIG. 1A, provides an input beam 140 including red (R)141, green (G) 142, and blue (B) 143 color channels, each at its owncorresponding center wavelength. The R color channel 141 is shown withsolid lines; the G color channel 142 is shown with short-dash lines; andthe B color channel 143 is shown with long-dash lines. The in-coupling101, 102, 103 and out-coupling 111, 112, 113 grating structures areconfigured to couple the R 141, G 142, and B 143 color channels in andout of the respective slabs 121, 122, and 123 such that only one colorchannel propagates in a corresponding slab 121, 122, and 123. In otherwords, the R color channel 141 propagates only or predominantly in thefirst slab 121, the G color channel 142 propagates only or predominantlyin the second slab 122, and the B color channel 143 propagates only orpredominantly in the third slab 123. To that end, the in-coupling 101,102, 103 and out-coupling 111, 112, 113 diffraction grating structuresare configured to merely pass through the other color channels,substantially without diffraction, i.e. without beam splitting orredirection. Since only one color channel propagates in any given slab121, 122, and 123, the overall viewing angle and wavelength performanceof the optics block 100B can be improved. An output beam 144, includingthe combined color channels 141, 142, and 143, propagates towards aneyebox 134. At least two pupil-replicating waveguides includingcorresponding slabs and gratings, but more commonly three waveguides,one for each color channel, may be provided.

Various configurations of diffraction gratings, which diffract light atonly one wavelength or at only one color channel, while propagatingthrough the remaining color channels, will now be described. Initially,it is noted that grooves of a diffractive structure can be representedby spatial oscillations of optical permittivity, i.e. permittivity atoptical frequencies, of a medium, such as thin layer of a material for athin diffraction grating. The optical permittivity spatially oscillateswith a certain pitch, i.e. a spatial period of oscillation, which can beconstant or varying. In accordance with the present disclosure, theamplitude of such spatial variation at a certain pitch, i.e. theamplitude of spatial oscillation of optical permittivity at a givenpitch, can be made wavelength-dependent; furthermore, for differentgrating pitches, the wavelength dependence of the amplitude isdifferent. For example, the in-coupling 101, 102, 103 and out-coupling111, 112, 113 grating structures can have wavelength-dependentamplitudes of spatial variations of optical permittivity at generallydifferent values of grating pitch for diffracting light at differentwavelengths.

The latter point is illustrated in FIG. 2A. A first amplitude 201 of aspatial variation of optical permittivity of the first in-coupling 101and out-coupling 111 grating structures (FIG. 1B) at a first gratingpitch is above a first threshold T₁ (FIG. 2A) at a first wavelength λ₁,and is below a second threshold T₂ at a second wavelength λ₂. A secondamplitude 202 of a spatial variation of optical permittivity of thesecond in-coupling 102 and out-coupling 112 grating structures at asecond grating pitch is below the first threshold T₁ at the firstwavelength λ₁, and is above the second threshold T₂ at the secondwavelength λ₂. In this manner, the first in-coupling 101 andout-coupling 111 grating structures can be made to predominantlydiffract light at the first wavelength λ₁, and the second in-coupling102 and out-coupling 112 grating structures can be made to predominantlydiffract light at the second wavelength λ₂. The different first andsecond grating pitches enable independent diffraction angle control fordifferent wavelengths. Herein, the first wavelength λ₁ may correspond tothe center wavelength of the R channel, and the second wavelength λ₂ maycorrespond to the center wavelength of the G channel. More generally,the first λ₁ and second λ₂ wavelengths can be any two different opticalwavelengths, i.e. wavelengths of visible light corresponding todifferent color channels of an electronic display, for an AR/VR displayapplication.

For visual display systems with three color channels, a third amplitude203 (FIG. 2B) of a spatial variation of optical permittivity of thethird in-coupling 103 and out-coupling 113 grating structures at a thirdgrating pitch is above the first threshold T₁ at a third wavelength λ₃corresponding to the blue (B) channel while being below the secondthreshold T₂ at the first λ₁ and second λ₂ wavelengths. In such asystem, at the third wavelength λ₃, the first amplitude 101 and thesecond amplitude 102 are both below the second threshold T₂.

In FIGS. 2A and 2B, the second threshold T₂ is lower than the firstthreshold T₁. The ratio T₂/T₁ of two thresholds will determine theleakage of light at “wrong” wavelength into a downstream diffractivegrating structure. It is generally desirable to decrease the T₂/T₁ratio, which depends on the manufacturing technology of the diffractivegrating structures with wavelength-dependent amplitude of permittivityvariation. The T₂/T₁ ratio can be below 10%, for example. It ispreferable to have the T₂/T₁ ratio of less than 1%, or even lower, e.g.less than 0.5% to reduce the formation of ghost images, which may beeasily picked by an eye, especially when the ghost images are in-focus.It is further noted that the optical permittivity can be a complexnumber including a real part corresponding to the refractive index, andan imaginary part corresponding to the absorption coefficient. Herein,the term “amplitude of permittivity” refers to the amplitude ofrefractive index, the absorption coefficient, or more generally to amodulus of a complex optical permittivity, i.e. a square root of the sumof refractive index squared and the absorption coefficient squared. Itis further noted that for the case where optical permittivity is a realnumber, the amplitude of spatial variation of permittivity correspondsto a value known as refractive index contrast, or simply index contrast,of a diffraction grating structure at a certain pitch. For such purelyrefractive grating structures, the first refractive index contrast atthe first wavelength λ₁ is above the first threshold T₁ and at thesecond wavelength λ₂, the first refractive index contrast is below thesecond threshold T₂. At the first wavelength λ₁, the second refractiveindex contrast is below the second threshold T₂ and at the secondwavelength λ₂, the second refractive index contrast is above the firstthreshold T₁. The third index contrast can also follow this rule, i.e.it is higher than the first threshold T₁ at the third wavelength λ₃, butlower than the second threshold T₂ at the two remaining wavelengths λ₁and λ₂.

A refractive index contrast spectrum of a purely refractive diffractiongrating of the present disclosure is illustrated in FIG. 2C. The firstin-coupling 101 and out-coupling 111 grating structures have awavelength-dependent index contrast 211; the second in-coupling 102 andout-coupling 112 grating structures have a wavelength-dependent indexcontrast 212; and the third in-coupling 103 and out-coupling 113 gratingstructures have a wavelength-dependent index contrast 213. The firstindex contrast 211 peaks at the wavelength of the first (R) colorchannel and is substantially zero at the second (G) and third (B)channel wavelengths; the second index contrast 212 peaks at the second(G) channel wavelength and is substantially zero at the first (R) andthird (B) channel wavelengths; and the third index contrast 213 peaks atthe third (B) channel wavelength and is substantially zero at the first(R) and third (B) channel wavelengths. As a result, the firstin-coupling 101 and out-coupling 111 grating structures aresubstantially transparent, i.e. invisible, for light at the G and Bchannel wavelengths; the second in-coupling 102 and out-coupling 112grating structures are substantially transparent for the R and B channelwavelengths; and the third in-coupling 103 and out-coupling 113 gratingstructures are substantially transparent for the R and G channelwavelengths, causing each color channel to travel strictly in the slabit is assigned to. In other words, the R color channel 141 travels onlyin the first slab 121, the G color channel 142 travels only in thesecond slab 122, and the B color channel 143 travels only in the thirdslab 123.

The in-coupling grating structures 101, 102, and 103 may be consideredas components of a compound in-coupling diffraction grating 150 (FIG.1B), and the out-coupling grating structures 111, 112, and 113 may beconsidered as components of a compound out-coupling diffraction grating160. In one embodiment, the in-coupling grating structures 101, 102, and103 can be combined on a single substrate, and the out-couplingstructures 111, 112, and 113 may be combined on a same or differentsubstrate. Referring to FIG. 3A, an optics block 300 for a near-eyedisplay (NED) or a head-mounted display (HMD) includes an in-couplingdiffraction grating 350 and an out-coupling diffraction grating 360,both supported by a slab 320. Turning to FIG. 3B, the in-couplinggrating 350 includes a first grating structure 351 for R channel; asecond grating structure 352 for G channel; and a third gratingstructure 353 for B channel. The grating structures 351-353 are disposedadjacent one another, and may be directly disposed on top of oneanother. Each grating structure has its own pitch d. The diffractionequation for the first order of diffraction may be written down assin θ=n(λ/d)  (1)

where θ is the angle of diffraction, λ wavelength, and n is refractiveindex of the surrounding medium. It follows from Eq. (1) that when theratio of pitches of the grating structures 351, 352, and 353 is equal tothe ratio of wavelengths of corresponding R 141, G 142, and B 143 colorchannels, the diffraction of all color components of the impingingoptical beam 140 will occur at substantially the same angle, enablingthe in-coupling grating 350 to redirect the R 141, G 142, and B 143color channels substantially in a same direction. In other words, thein-coupling grating 350 can redirect the impinging optical beam 140 at adesired angle, i.e. a shallow angle as needed, substantially withoutcolor separation or color dispersion. The out-coupling grating 360 maybe constructed in a similar manner. The in-coupling 350 and out-coupling360 gratings may be purely refractive as illustrated in FIG. 2C, or moregenerally they may have complex optical permittivities as illustrated inFIGS. 2A and 2B. It is further noted that the in-coupling 350 andout-coupling 360 gratings may be used in place of the correspondinggratings 181 and 182 of the optics block 100A of FIG. 1A, resulting inan improvement of wavelength range, viewing angle range, or both, of theoptics block 100A.

In accordance with one aspect of the present disclosure, a diffractiongrating for beam coupling, redirecting, and/or compressing a circularspot beam into an elliptical-spot beam can include materials with aspatially varying optical permittivity dependent on wavelength in such amanner that the grating structures 351, 352, and 353 are implemented ina same layer of material, resulting in a novel diffraction grating withunique properties. Referring to FIG. 4, a desired optical permittivityε(λ,x) is illustrated in a 3D spectral-spatial diagram. At a firstwavelength, e.g. the center wavelength λ_(R) of R color channel, thespatially varying optical permittivity modulus |ε(λ,x)| comprises aplurality of peaks 401 and valleys 402 at a first pitch d₁. At a second,different wavelength, e.g. the center wavelength λ_(G) of G colorchannel, the spatially varying optical permittivity |ε(λ,x)| comprises aplurality of peaks 411 and valleys 412 at a second, different pitch d₂.Optionally, at a third, different wavelength, e.g. the center wavelengthof B color channel, the spatially varying optical permittivity |ε(λ,x)|comprises a plurality of peaks 421 and valleys 422 at a third, differentpitch d₃. This allows one to independently control the diffractionangles at different wavelengths, in a similar manner as the compoundin-coupling 350 and out-coupling 360 diffraction gratings describedabove, only in this case, it is a same optical medium that has opticalproperties of a stack of three different gratings. For example, when theratio of pitches d₁/d₂/d₃ is equal to the ratio of center wavelengths ofcorresponding R, G, B color channels λ_(R)/λ_(G)/λ_(B), the diffractionof all color components of an impinging optical beam 500 (FIG. 5) willoccur at the same angle θ, enabling the diffraction grating to redirectthe R 501, G 502, and B 503 channels sub-beams in a same direction.While the magnitude of the spatially varying optical permittivity ε(λ,x)depends on implementation technologies and materials used, it may beadvantageous to use optically transparent materials with purely realoptical permittivity ε(λ,x), i.e. where the spatially varying opticalpermittivity ε(λ,x) is dominated by a spatially varying refractiveindex. Such gratings are not absorptive and may have a higher overalldiffraction efficiency.

Non-limiting examples of implementations of the above describeddiffraction gratings will now be considered. Referring to FIG. 6A, adiffraction grating 600A includes first 601 and second 602 gratingstructures. The first grating structure 601 includes a plurality offirst embedded nanoparticles 611 at a first spatially varying density,e.g. in blocks 621 at a first pitch d₁, as shown. The firstnanoparticles 611 (shown as white spheres) have a surface plasmonresonance at the first wavelength, e.g. the center wavelength of the Rchannel λ_(R). A surface plasmon resonance occurs when the frequency ofthe external optical field is close to a resonant frequency of surfaceplasmon waves, i.e. surface electromagnetic waves, of a nanoparticle,causing the nanoparticle to act as a resonant nanoantenna forelectromagnetic radiation at light frequencies. This antenna resonanceeffect results in a significant enhancement of the light absorption andemission cross-section of the nanoparticle. As in case of a regularantenna, the resonant frequency depends on the antenna size and shape,enabling fine tuning of such resonance. The first 611 and second 612nanoparticles may be spherical, ellipsoidal, rod-like, etc.

The second grating structure 602 includes a plurality of second embeddednanoparticles 612 at a second spatially varying density, e.g. in blocks622 at a second, different pitch d₂, as shown. The second nanoparticles612 (shown as black spheres) have a surface plasmon resonance at thesecond, different wavelength, e.g. the center wavelength of the Gchannel λ_(G) of an electronic display. To that end, the first 611 andsecond 612 nanoparticles may have different outer diameters (forspherical particles), may be of different materials such as gold (Au)and silver (Ag), may include a core made of a different material such assemiconductor or dielectric, etc. The latter configuration is termedherein a “core shell” nanoparticle configuration. The core shellnanoparticles may also be of different diameters and core-shell fillratios. For example, the first nanoparticles may includemetal-semiconductor core shells of a first diameter, and the secondnanoparticles may include metal-semiconductor core shells of a seconddiameter. Alternatively, the first nanoparticles may includemetal-dielectric core shells of a first diameter, and the secondnanoparticles may include metal-dielectric core shells of a seconddiameter. A third grating structure for B channel may also be providedwith third nanoparticles varying by diameters, composition, fill ratios,and the like.

In operation, the first nanoparticles 611 scatter light at the R channelwavelengths due to the surface plasmon resonance, while the scatteringat the G and B channel wavelengths is zero or negligibly small. Sincethe nanoparticles are disposed in blocks 621, 622 having a grating-likeconfiguration, together they act as lines of a diffraction grating,causing a diffraction of the R channel at a pre-determined angle definedby the grating equation (1) above, while being substantially transparentand/or non-diffracting for the G and B channels. Similarly, the secondnanoparticles 612 only scatter and diffract light of the G channel; andthe third nanoparticles only scatter and diffract light of the B channelof an electronic display. By way of a non-limiting example, thediffraction grating 600A may be used for the in-coupling grating 350 ofFIGS. 3A, 3B and/or the out-coupling grating 360 of FIG. 3A. In FIG. 6B,a diffraction grating 600B includes first 601 and second 602 gratingstructures superimposed in a common layer 620, i.e. the first 611 andsecond 612 nanoparticles are embedded in the same layer 620. In thisembodiment, the first grating structure 601 includes an array of firststripes of the first nanoparticles 611 at the first grating pitch, theand the second grating structure 602 includes an array of second stripesof nanoparticles 612 at the second, different grating pitch. Similarly,a third grating structure, omitted for brevity in FIG. 6B, may includean array of third stripes of nanoparticles at the third grating pitch,and so on. When different nanoparticles are embedded in a same layer,the resulting optical permittivity may have a multi-wavelengthresonance. In other words, at one wavelength, the spatially varyingoptical permittivity ε(λ,x) comprises a plurality of peaks and valleysat a first pitch, and at another, different wavelength, the spatiallyvarying optical permittivity ε(λ,x) comprises a plurality of peaks andvalleys at a second, different pitch, and so forth. Thismulti-wavelength, spatially varying permittivity ε(λ,x) has beenillustrated in FIG. 4 for three center wavelengths of three colorchannels: red (R), green (G), and blue (B).

Turning to FIG. 6C, a diffraction grating 600C is shown in a top view.The diffraction grating 600C is similar to the diffraction grating 600Bof FIG. 6B, only in FIG. 6C, the first 611 and second 612 nanoparticlesare adhered to a same upper surface of the layer 620, i.e. the layer 620functions as a substrate layer for the first 611 and second 612nanoparticles. This provides a two-wavelength resonance of theamplitudes of the corresponding peaks and valleys of the spatiallyvarying optical permittivity ε(λ,x). In FIG. 6C, the first gratingstructure 601 includes an array of first stripes of the firstnanoparticles 611 at the first grating pitch, the and the second gratingstructure 602 includes an array of second stripes of the secondnanoparticles 612 at the second, different grating pitch. By providing athird stripe of third nanoparticles having a plasmon resonance at athird, different wavelength, a three-wavelength resonance of theamplitudes of the corresponding peaks and valleys of the spatiallyvarying optical permittivity ε(λ,x) may be obtained, as has beenexplained above with reference to FIG. 4. The three wavelengths can beselected to be close to centers of the R, G, and B color channels, whichenables the diffraction grating 600C to redirect the R, G, and B colorchannels independently on each other, including diffracting the R, G,and B color channels in a same direction if so desired.

Referring now to FIG. 6D, a diffraction grating 600D is shown in topview. The diffraction grating 600D is similar to the diffraction grating600C of FIG. 6C, only in FIG. 6D, the nanoparticles with differentplasmon resonance wavelengths are defined using microfabrication methodsbased on photolithography, and are disposed in a repeating geometricalpattern, i.e. in an interlaced diamond pattern in FIG. 6D. Only twotypes of nanoparticles are shown for brevity. The first nanoparticles631 are shown as hexagons, and the second nanoparticles 632 are shown asstars. The stripes of first nanoparticles 631 form a first sub-array 641at a first pitch i.e. the period of stripes, and the stripes of secondnanoparticles 632 form a second sub-array 642 at a second, larger pitchi.e. the period of stripes, creating a two-wavelength resonant spatiallyvarying effective permittivity ε(λ,x). It is to be understood that thestar and hexagon shaped nanoparticles are only examples, and a varietyof lithographically defined nanoparticle shapes/sizes/compositions arepossible, enabling one to engineer the wavelength and polarizationsensitivity of the gratings formed by the nanoparticle sub-arrays.

Photolithography and other microfabrication methods may be used not onlyto manufacture the diffraction grating 600D of FIG. 6D, but also tomanufacture other types of plasmon-resonance based, wavelength-selectivediffraction gratings. By way of example, referring to FIGS. 7A-7F, aprocess of manufacturing the diffraction grating 600C of FIG. 6C isillustrated. A substrate 700 (FIG. 7A) is coated with a photoresist 702(FIG. 7B) using any suitable technique such as spin coating, forexample. The photoresist 702 is then patterned (FIG. 7C) according tothe desired diffraction grating structure using suitablephotolithographic techniques, e.g. by exposing the photoresist to UVlight through a pre-configured mask, and stripping or removing unexposedphotoresist which was not photo-polymerized by the UV light. A solution704 is then applied (FIG. 7D) that alters the hydrophilic/hydrophobicproperties of the substrate 700. In this example, the substrate 700 ishydrophobic and the solution makes exposed areas of the substrate 700hydrophilic, although a reverse scenario, i.e. a hydrophilic substrateand hydrophobic solution, is also possible. Then, a colloidal solution706 of nanoparticles, e.g. silver nanoparticles, is applied (FIG. 7E),causing the nanoparticles to adhere to exposed areas 708 of substrate(FIG. 7F), which were made hydrophilic (FIG. 7D). The remainingphotoresist 702 may then be stripped away.

Turning to FIGS. 8A to 8D, various nanoparticles may be provided invarious configurations including but not limited to stars (FIG. 8A),hexagons (FIG. 8B), compound spheres or core shells (FIG. 8C) andordered, or arrayed structures (FIG. 8D). Both ordered and non-ordered,i.e. pseudo-random, nanoparticle configurations and structures may bedefined lithographically.

Other types of gratings can be provided. According to one embodiment, adiffraction grating having different grating pitches for differentwavelengths may be constructed using metamaterials. A metamaterialstructure can include a stack of thin layers of metal, semiconductor,and/or insulator, patterned to form an array of stripes at a desiredpitch. Referring to FIG. 9, a metamaterial diffraction grating 900includes a substrate 902 and a patterned hyperbolic metamaterial layer904 supported by the substrate 902 and including a stack 908 of silver(Ag; 910) and silicon (Si; 912) etched through to the substrate 902 toconstruct an array of stripes 914, which act as diffraction gratinggrooves for diffracting an incoming optical beam 920 to produce adiffracted optical beam 922. The array of stripes 914 can be backfilledwith an index-matching material 915.

The optical properties of the stack 908 depend on Ag/Si fill ratio. Thebulk optical permittivity of Ag and Si is shown in FIG. 10A. The realpart of Ag permittivity Re(ε_(Ag)) 1002 is shown with a thick solidline, the imaginary part of Ag permittivity Im(ε_(Ag)) 1004 is shownwith a thick dashed line, the real part of Si permittivity Re(ε_(Si))1006 is shown with a thin solid line, and the imaginary part of Sipermittivity Im(ε_(Si)) 1008 is shown with thin dashed line. Theengineered metamaterial permittivity is shown in FIG. 10B for opticalbeam polarization 924 (FIG. 9) in plane of incidence marked with “⊥”sign. The real part of the metamaterial permittivity Re(ε_(⊥)) 1010(FIG. 10B) is shown with a thick solid line, the imaginary part f themetamaterial permittivity Im(ε_(⊥)) 1012 is shown with a thick dashedline, and permittivity ε_(match) 1014 of the matching material 915 isshown with a thin dotted line. One can see from FIG. 10B that the realpart of the permittivity Re(ε_(⊥)) 1010, corresponding to the refractiveindex, may be matched to the permittivity of the surrounding mediumε_(match) 1014, i.e. the matching material 915 in this case, essentiallycausing the grating to “disappear” for an optical beam at the indexmatching wavelength.

The latter point is further illustrated in FIG. 11. The gratingpermittivity 1102 crosses the surrounding medium permittivity 1104 at acrossing point 1100. At a wavelength of the crossing point 1100 betweenthe grating permittivity 1102 and surrounding medium permittivity 1104,a permittivity contrast curve 1106 has a null, i.e. goes down to zero.The nulling point effectively “erases” the grating for the light at theindex matching wavelength. Accordingly, the light at the index matchingwavelength will not be diffracted at all.

The nulling effect of a hyperbolic metamaterial grating may be used toconstruct a compound diffraction grating, in which the diffraction oflight at two different wavelength is independently controlled. Referringto FIG. 12, a diffraction grating 1200 includes first 1201 and second1202 grating structures supported by a substrate 1240. The first 1201and second 1202 grating structures operate at first and second differentwavelengths. Each one of the first 1201 and second 1202 gratingstructures includes its own hyperbolic metamaterial grating structure,with different pitches of the metamaterial stripes, and, optionally,with different metamaterial compositions. In FIG. 12, the first gratingstructure 1201 has a larger pitch than the second grating structure1202. The Ag/Si fill ratio can be selected such as to null out therefractive index contrast, and accordingly, the diffraction efficiency,at two different wavelengths, as shown in FIG. 13. The first gratingstructure 1201 has a null point 1301 at the second wavelength, i.e.working wavelength of the second grating structure 1202, and vice versa:the second grating structure 1202 has a null point 1302 at the firstwavelength. This allows independent control of diffraction of the lightbeam 920 at the two wavelengths, making it possible to equate thediffraction angle of the diffracted optical beam 922 for the twowavelengths, if required. The diffraction grating 1200, taken as awhole, can be described by the spatially varying, wavelength-dependentpermittivity illustrated in FIG. 4 above, where at the first wavelength,the spatially varying optical permittivity comprises a plurality ofpeaks and valleys at a first pitch, and at the second, differentwavelength, the spatially varying optical permittivity comprises aplurality of peaks and valleys at a second, different pitch.

Other material combinations than Ag/Si may be used. By way ofnon-limiting examples, each stripe of the first grating structure 1201can include a stack of alternating metal and semiconductor layers, ormetal and dielectric layers, having a first set of thicknesses toprovide a zero refractive index contrast at the second wavelength. Eachstripe of the second grating structure can include a stack ofalternating metal and semiconductor layers, or metal and dielectriclayers, or both, having a second set of thicknesses to provide a zerorefractive index contrast at the first wavelength. A ratio of the firstto second grating pitches can be made equal to a ratio of the first tosecond wavelengths. As follows from the grating equation (1) above,components of an optical beam at the first and second wavelengthsimpinging on such diffraction grating will be diffracted at asubstantially same angle.

The diffraction grating structures described above, includingplasmon-resonant gratings and metamaterials gratings, can be used toconstruct optical waveguides for near-eye displays, for example thepupil-replicating waveguide 174 of the optics block 100A of FIG. 1A; thegrating structures 101/111 on the slab 121, the grating structures102/112 on the slab 122, the grating structures 103/113 on the slab 123,of the optics block 100B of FIG. 1B; or the grating structures 350/360on the slab 320 of the optics block 300 of FIG. 3A.

Referring to FIG. 14A with further reference to FIG. 2A, an opticalwaveguide 1400A for a near-eye display includes a slab 1420 oftransparent material and a first grating structure 1451 supported by theslab 1420. The first grating structure 1451 has a first spatialvariation of optical permittivity having a first grating pitch and awavelength-dependent first amplitude 201 (FIG. 2A), such that at thefirst wavelength λ₁, the first amplitude 201 is above the firstthreshold T₁ and at the second wavelength λ₂, the first amplitude 201 isbelow the second threshold T₂. A second grating structure 1452 issupported by the first grating structure 1451. The second gratingstructure 1452 has the second spatial variation of optical permittivityhaving the second grating pitch and the wavelength-dependent secondamplitude 202, such that at the first wavelength λ₁, the secondamplitude 202 is below the second threshold T₂ and at the secondwavelength λ₂, the second amplitude is above the first threshold T₁(FIG. 2A).

The optical permittivity of the first grating structure 1451 can includea spatially varying real portion, i.e. a refractive index having awavelength-dependent first refractive index contrast, and the opticalpermittivity of the second grating structure 1452 can include aspatially varying real portion, i.e. a refractive index having awavelength-dependent second refractive index contrast such as shown inFIG. 2C: at the first wavelength λ₁, the first refractive index contrast211 is above the first threshold T₁ and at the second wavelength λ₂, thefirst refractive index contrast 211 is below the second threshold T₂.Further, as shown in FIG. 2C, at the first wavelength λ₁, the secondrefractive index contrast 212 is below the second threshold T₂ and atthe second wavelength λ₂, the second refractive index contrast 212 isabove the second threshold T₂.

Still referring to FIG. 14A with further reference now to FIGS. 2B and2C, a third grating structure 1453 may be provided. The third gratingstructure 1453 may be supported by the second grating structure 1452 andmay have a third spatial variation of optical permittivity. The thirdspatial variation has a third grating pitch and a wavelength-dependentthird amplitude. At the first λ₁ and second λ₂ wavelengths, the thirdamplitude 203 is below the second threshold T₂ (FIG. 2B) and at a thirdwavelength λ₃, the third amplitude 203 is above the first threshold T₁.Further, at the third wavelength λ₃, the first amplitude 201 and thesecond amplitude 202 are both below the second threshold T₂. Each one ofthe first λ₁, second λ₂, and third λ₃ wavelengths may correspond to acolor channel of an electronic display, e.g. the electronic display 176of FIG. 1A. In embodiments where the diffraction angle for the differentcolor channels is the same as illustrated in FIG. 5, ratios of the firstgrating pitch to the first wavelength λ₁; the second grating pitch tothe second wavelength λ₂; and the third grating pitch to the thirdwavelength λ₃ can be made equal to each other, causing the individualcolor channel sub-beams 501, 502, and 503 to co-propagate.

The grating structures 1451, 1452, and 1453 may be implemented by usingvarious technologies and material systems. In one embodiment, thegrating structures 1451, 1452, and 1452 may include resonantnanoparticles of different surface plasmon resonant wavelengths oroptical frequencies, which may correspond to different color channels ofan electronic display. The nanoparticles of variousshapes/sizes/compositions, e.g. as shown in FIGS. 8A-8D, may be disposedat different spatially varying densities to form respective gratinggroove structures, e.g. as shown in FIGS. 6A-6D.

In one embodiment, the first grating structure 1451 may include a firsthyperbolic metamaterial comprising the array of first stripes at thefirst grating pitch, and the second grating structure 1452 may include asecond hyperbolic metamaterial comprising an array of second stripes atthe second grating pitch, similarly to the first grating structure 1201and the second grating structure 1202 of FIG. 12. The array of firststripes can have a zero refractive index contrast at the secondwavelength, and the array of second stripes can have zero refractiveindex contrast at the first wavelength, as illustrated in FIG. 13. Eachfirst stripe can include a stack of alternating metal and semiconductorlayers, or metal and insulating layers, or both, having a first set ofthicknesses to provide a zero refractive index contrast at the secondwavelength. Similarly, each second tripe can include a stack ofalternating metal and semiconductor layers having a second set ofthicknesses to provide a zero refractive index contrast at the firstwavelength. In both resonant nanoparticle and metamaterial gratingembodiments, the period or pitch of the stripes may be selected tocontrol the diffraction angles of the R, G, and B color channelsindependently of one another. For example, all three diffraction anglesmay be selected to be equal to each other.

Turning to FIG. 14B with further reference to FIG. 4, an opticalwaveguide 1400B for a near-eye display includes the slab 1420 oftransparent material and a diffraction grating 1460 supported by theslab and including a layer of material having the spatially varyingoptical permittivity ε(λ,x) dependent on wavelength as illustrated inthe 3D spectral-spatial diagram of FIG. 4. At the first (R channel)wavelength λ₁, the spatially varying optical permittivity ε(λ,x)comprises a plurality of peaks 401 and valleys 402 at a first spatialperiod or pitch d₁. At the second (G channel) wavelength λ₂, thespatially varying optical permittivity ε(λ,x) comprises a plurality ofpeaks 411 and valleys 412 at a second, different spatial period or pitchd₂. Furthermore, at the third (B channel) wavelength λ₂, the spatiallyvarying optical permittivity ε(λ,x) may include a plurality of peaks 421and valleys 422 at a first spatial period or pitch d₃. For phasegratings, the spatially varying optical permittivity ε(λ,x) comprises aspatially varying refractive index n(λ,x) corresponding to the real partof the spatially varying optical permittivity ε(λ,x). For amplitudegratings, the spatially varying optical permittivity ε(λ,x) comprises aspatially varying absorption coefficient α(λ,x), corresponding to theimaginary part of the spatially varying optical permittivity ε(λ,x). Tomake sure that components of an optical beam at the first λ₁ and secondλ₂ wavelengths impinging on the diffraction grating 1460 are diffractedat a substantially same diffraction angle as shown in FIG. 5, a ratio ofthe first d₁ to second d₂ pitches can be made equal to a ratio of thefirst λ₁ to second λ₂ wavelengths. Similarly, a ratio of the first d₁ tothird d₃ pitches can be made equal to a ratio of the first λ₁ to thirdλ₃ wavelengths.

The diffraction grating 1460 may be constructed using varioustechnologies and material systems. In one embodiment, the diffractiongrating 1460 may include a plurality of first nanoparticles at a firstspatially varying density, having a surface plasmon resonance at thefirst wavelength λ₁, and a plurality of second nanoparticles at a secondspatially varying density, having a surface plasmon resonance at thesecond wavelength λ₂. The nanoparticles of variousshapes/sizes/compositions, e.g. as shown in FIGS. 8A-8D, may be disposedat different spatially varying densities to form respective gratinggroove structures, e.g. as shown in FIGS. 6C and 6D. By way of anon-limiting example, the plurality of first nanoparticles may includean array of first stripes of nanoparticles at the first pitch, and theplurality of second nanoparticles may include an array of second stripesof nanoparticles at the second pitch.

Still referring to FIG. 14B with further reference now to FIGS. 12 and13, the diffraction grating 1460 may also include a hyperbolicmetamaterial. The hyperbolic metamaterial may include an array of firststripes at the first pitch (the first grating structure 1201 in FIG. 12)and having a zero refractive index contrast at the second wavelength(1302 in FIG. 13), and an array of second stripes at the second pitch(the second grating structure 1202 in FIG. 12) and having a zerorefractive index contrast at the first wavelength (1301 in FIG. 13).Each first stripe can include a stack of alternating metal andsemiconductor layers, metal and insulator layers, or both, having afirst set of thicknesses to provide a zero refractive index contrast atthe second wavelength λ₂, and each second tripe can include a stack ofalternating metal and semiconductor layers, metal and insulator layers,or both, having a second set of thicknesses to provide a zero refractiveindex contrast at the first wavelength λ₁. In both resonant nanoparticleand metamaterial grating embodiments, the period or pitch of the stripesmay be selected to control the diffraction angles of the R, G, and Bcolor channels independently on one another. For example, all threediffraction angles may be selected to be equal to each other asillustrated in FIG. 5.

The diffraction gratings described above can be used in an optics blockof a near-eye display, such as the optics block 100A of FIG. 1A or FIG.15. Referring specifically to FIG. 15, a near-eye display 1500 includesa body 1580, a pupil-replicating optical waveguide 1574, an electronicdisplay 1576 for providing image light 1588, and collimating opticsrepresented by a lens 1578 for collimating the image light 1588. Theimage light 1588 may contain at least two color channels, e.g. first andsecond color channels, three color channels, e.g. first, second, andthird color channels, or more. The electronic display 1576 and the lens1578 are parts of a projector 1572. The body 1580 may be of differentshapes, from a simple sunglasses frame to a complete enclosure to beworn on a user's head.

The pupil-replicating optical waveguide 1574 is configured to receiveand guide therein the image light 1588 from the electronic display 1576.To that end, the optical waveguide 1574 may include a slab 1575 oftransparent material such as glass or plastic, a first diffractiongrating 1581 supported by the slab 1575 and configured for receiving theimage light 1588 from the electronic display 1576 for coupling into theslab 1575. A second diffraction grating 1582 is supported by the sameslab 1575, and is configured for receiving the optical beam 188 from thefirst diffraction grating 1581 through the slab 1575 for out-couplingoutput light 1589 from the slab 1575. In one embodiment, each one of thefirst 1581 and second 1582 diffraction gratings includes a layer ofmaterial having a spatially varying optical permittivity dependent onwavelength shown in FIG. 4. At the first wavelength λ₁ of the firstcolor channel, the spatially varying optical permittivity ε(λ,x)comprises a plurality of peaks and valleys at a first spatial period(pitch), and at the second wavelength λ₂ of the second color channel,the spatially varying optical permittivity ε(λ,x) comprises a pluralityof peaks and valleys at a second, different spatial period (pitch). Incases where the permittivity ε(λ,x) is represented by a real number, theamplitude of the permittivity variation corresponds to a refractiveindex contrast of the grating structure, which is wavelength-dependent,such that at different wavelengths, the grating has different pitch.Alternatively or in addition, each one of the first 1581 and second 1582diffraction gratings may also include a plurality of grating structures,each for diffracting light at a particular color channel, while beingsubstantially transparent, i.e. showing no diffraction, at thewavelengths of the remaining color channels. The amplitude of spatialvariation of permittivity of such gratings is schematically illustratedin FIGS. 2A-2C. In any of the above embodiments, the period or pitch ofthe spatial variations of optical permittivity ε(λ,x) or refractiveindex n(λ,x) may be selected to control the diffraction angles ofdifferent color channels, i.e. the R, G, and B color channels,independently on one another.

By way of a non-limiting example, all three diffraction angles may beselected to be equal to each other as illustrated in FIG. 5. Inoperation, components of the image light 1588 (FIG. 15) at the first λ₁and second λ₂ wavelengths impinging on the first diffraction grating1581 are diffracted at a substantially same angle for joint propagationin the slab 1575. Components of the image light 1588 at the first λ₁ andsecond λ₂ wavelengths impinging on the second diffraction grating 1582are diffracted at a substantially same angle for joint propagationtowards an eye of a user. The first 1581 and second 1582 diffractiongratings may also be configured to operate with three or more colorchannels. For example, the first wavelength λ₁ can correspond to the red(R) color channel, the second wavelength λ₂ can correspond to the green(G) color channel, and the third wavelength λ₃ can correspond to theblue (B) color channel. The amplitude and the spatial period (pitch) ofspatial oscillations of the permittivity |ε(λ,x)| of the diffractiongratings 1581 and 1582 can be wavelength-dependent, as shown in FIG. 4,for independent control of the diffraction angle of each color channel.

The diffraction gratings and optics blocks described herein may be usedto provide multi-wavelength light guiding, directing, and/or couplingfor an HMD in an efficient, space-saving manner. Referring to FIGS. 16Aand 16B, a near-eye artificial reality (AR)/virtual reality (VR) display1600 is an embodiment of the near-eye display 1500 of FIG. 15, and caninclude the optics block 100A of FIG. 1A, the optics block 100B of FIG.1B, and/or the optics block 300 of FIG. 3A. A body or frame 1602 of thenear-eye AR/VR display 1600 has a form factor of a pair of eyeglasses,as shown. A display 1604 includes a display assembly 1606, for example,the optics block 100A of FIG. 1A or the optics block 100B of FIG. 1B.The display assembly 1606 (FIG. 16) provides image light 1608 to aneyebox 1610, i.e. a geometrical area where a good-quality image may bepresented to a user's eye 1612. The display assembly 1606 may include aseparate VR/AR display module for each eye, or one VR/AR display modulefor both eyes. For the latter case, an optical switching device may becoupled to a single electronic display for directing images to the leftand right eyes of the user in a time-sequential manner, one frame forleft eye and one frame for right eye. The images are presented fastenough, i.e. with a fast enough frame rate, that the individual eyes donot notice the flicker and perceive smooth, steady images of surroundingvirtual or augmented scenery. An electronic display of the displayassembly 1606 may include, for example and without limitation, a liquidcrystal display (LCD), an organic light emitting display (OLED), aninorganic light emitting display (ILED), an active-matrix organiclight-emitting diode (AMOLED) display, a transparent organic lightemitting diode (TOLED) display, a projector, or a combination thereof.More generally, such a display may be provided for any of the displaymodules or systems disclosed herein. The near-eye AR/VR display 1600 mayalso include an eye-tracking system 1614 for determining, in real time,the gaze direction and/or the vergence angle of the user's eyes 1612.The determined vergence angle may then be used to obtain the Dioptervalue of the display's varifocal lenses for lessening thevergence-accommodation conflict. The determined gaze direction andvergence angle may also be used for real-time compensation of visualartifacts dependent on the angle of view and eye position, such asluminance uniformity, color uniformity, pupil swim, etc. Furthermore,the determined vergence and gaze angles may be used for interaction withthe user, highlighting objects, bringing objects to the foreground,dynamically creating additional objects or pointers, etc. Furthermore,the near-eye AR/VR display 1600 may include an audio system, such assmall speakers or headphones.

Turning now to FIG. 17, an HMD 1700 is an example of an AR/VR wearabledisplay system which encloses the user's face, for a greater degree ofimmersion into the AR/VR environment. The HMD 1700 can present contentto a user as a part of an AR/VR system, which may further include a userposition and orientation tracking system, an external camera, a gesturerecognition system, control means for providing user input and controlsto the system, and a central console for storing software programs andother data for interacting with the user for interacting with the AR/VRenvironment. The function of the HMD 1700 is to augment views of aphysical, real-world environment with computer-generated imagery, and/orto generate the entirely virtual 3D imagery. The HMD 1700 may include afront body 1702 and a band 1704. The front body 1702 is configured forplacement in front of eyes of a user in a reliable and comfortablemanner, and the band 1704 may be stretched to secure the front body 1702on the user's head. A display system 1780 may include the optics block100A of FIG. 1A, the optics block 100B of FIG. 1B, the optics block 300of FIG. 3A, the near-eye display 1500 of FIG. 15, which may include anyof the diffraction gratings described herein. The display system 1780may be disposed in the front body 1702 for presenting AR/VR imagery tothe user. Sides 1706 of the front body 1702 may be opaque ortransparent.

In some embodiments, the front body 1702 includes locators 1708, aninertial measurement unit (IMU) 1710 for tracking acceleration of theHMD 1700, and position sensors 1712 for tracking position of the HMD1700. The locators 1708 are traced by an external imaging device of avirtual reality system, such that the virtual reality system can trackthe location and orientation of the entire HMD 1700. Informationgenerated by the IMU and the position sensors 1712 may be compared withthe position and orientation obtained by tracking the locators 1708, forimproved tracking of position and orientation of the HMD 1700. Accurateposition and orientation is important for presenting appropriate virtualscenery to the user as the latter moves and turns in 3D space.

The HMD 1700 may further include an eye tracking system 1714, whichdetermines orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes allows the HMD 1700 todetermine the gaze direction of the user and to adjust the imagegenerated by the display system 1780 accordingly. In one embodiment, thevergence, that is, the convergence angle of the user's eyes gaze, isdetermined. The determined gaze direction and vergence angle may also beused for real-time compensation of visual artifacts dependent on theangle of view and eye position, such as luminance uniformity, coloruniformity, pupil swim, etc. Furthermore, the determined vergence andgaze angles may be used for interaction with the user, highlightingobjects, bringing objects to the foreground, creating additional objectsor pointers, etc. An audio system may also be provided including e.g. aset of small speakers built into the front body 1702.

The hardware used to implement the various illustrative logics, logicalblocks, modules, and circuits described in connection with the aspectsdisclosed herein may be implemented or performed with a general purposeprocessor, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor maybe a microprocessor, but, in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices,e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration. Alternatively, some steps ormethods may be performed by circuitry that is specific to a givenfunction.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth of thepresent disclosure as described herein.

What is claimed is:
 1. A diffraction grating comprising: a first gratingstructure having a first spatial variation of optical permittivity,wherein the first spatial variation has a first grating pitch and awavelength-dependent first amplitude, such that at a first wavelength,the first amplitude is above a first threshold and at a secondwavelength, the first amplitude is below a second threshold, wherein thesecond threshold is lower than the first threshold; and a second gratingstructure having a second spatial variation of optical permittivity,wherein the second spatial variation has a second grating pitch and awavelength-dependent second amplitude, such that at the firstwavelength, the second amplitude is below the second threshold and atthe second wavelength, the second amplitude is above the firstthreshold.
 2. The diffraction grating of claim 1, wherein the secondthreshold is below 10% of the first threshold.
 3. The diffractiongrating of claim 1, wherein the optical permittivity of the firstgrating structure comprises a spatially varying refractive index havinga wavelength-dependent first refractive index contrast, and wherein theoptical permittivity of the second grating structure comprises aspatially varying refractive index having a wavelength-dependent secondrefractive index contrast; wherein at the first wavelength, the firstrefractive index contrast is above the first threshold and at the secondwavelength, the first refractive index contrast is below the secondthreshold; and wherein at the first wavelength, the second refractiveindex contrast is below the second threshold and at the secondwavelength, the second refractive index contrast is above the firstthreshold.
 4. The diffraction grating of claim 3, wherein the secondthreshold is below 10% of the first threshold.
 5. The diffractiongrating of claim 4, wherein at the second wavelength, the firstrefractive index contrast is substantially zero, and wherein at thefirst wavelength, the second refractive index contrast is substantiallyzero.
 6. The diffraction grating of claim 1, further comprising a thirdgrating structure having a third spatial variation of opticalpermittivity, wherein the third spatial variation has a third gratingpitch and a wavelength-dependent third amplitude, such that at the firstand second wavelengths, the third amplitude is below the secondthreshold and at a third wavelength, the third amplitude is above thefirst threshold, wherein at the third wavelength, the first amplitudeand the second amplitude are both below the second threshold, andwherein each one of the first, second, and third wavelengths correspondsto a color channel of an electronic display.
 7. The diffraction gratingof claim 6, wherein ratios of the first grating pitch to the firstwavelength; the second grating pitch to the second wavelength; and thethird grating pitch to the third wavelength are equal to each other,such that in operation, components of an optical beam at the first,second, and third wavelengths emitted by the electronic display andimpinging on the diffraction grating are diffracted at a substantiallysame diffraction angle.
 8. The diffraction grating of claim 1, whereinthe first and second grating structures are disposed adjacent oneanother.
 9. The diffraction grating of claim 1, wherein the firstgrating structure comprises a plurality of first nanoparticles at afirst spatially varying density of nanoparticles, the firstnanoparticles having a surface plasmon resonance at the firstwavelength; and wherein the second grating structure comprises aplurality of second nanoparticles at a second spatially varying densityof nanoparticles, the second nanoparticles having a surface plasmonresonance at the second wavelength.
 10. The diffraction grating of claim9, wherein the first nanoparticles comprise metal spheres of a firstdiameter, and wherein the second nanoparticles comprise metal spheres ofa second diameter.
 11. The diffraction grating of claim 9, wherein thefirst nanoparticles comprise metal-semiconductor core shells of a firstdiameter, and wherein the second nanoparticles comprisemetal-semiconductor core shells of a second diameter.
 12. Thediffraction grating of claim 9, wherein the first nanoparticles comprisemetal-dielectric core shells of a first diameter, and wherein the secondnanoparticles comprise metal-dielectric core shells of a seconddiameter.
 13. The diffraction grating of claim 9 further comprising athird grating structure comprising a plurality of third nanoparticles ata third spatially varying density of nanoparticles, the thirdnanoparticles having a surface plasmon resonance at a third wavelength,wherein each one of the first, second, and third wavelengths correspondsto a color channel of an electronic display.
 14. The diffraction gratingof claim 13, wherein the plurality of first nanoparticles comprises anarray of first stripes of nanoparticles at the first grating pitch, theplurality of second nanoparticles comprises an array of second stripesof nanoparticles at the second grating pitch, and the plurality of thirdnanoparticles comprises an array of third stripes of nanoparticles atthe third grating pitch.
 15. The diffraction grating of claim 14,wherein ratios of the first grating pitch to the first wavelength; thesecond grating pitch to the second wavelength; and the third gratingpitch to the third wavelength are equal to each other, such that inoperation, components of an optical beam at the first, second, and thirdwavelengths emitted by the electronic display and impinging on thediffraction grating are diffracted at a substantially same diffractionangle.
 16. The diffraction grating of claim 14, wherein each stripecomprises a sub-array of nanoparticles.
 17. A diffraction gratingcomprising a layer of material with a spatially varying opticalpermittivity dependent on wavelength, such that: at a first wavelength,the spatially varying optical permittivity comprises a plurality ofpeaks and valleys at a first pitch; and at a second, differentwavelength, the spatially varying optical permittivity comprises aplurality of peaks and valleys at a second, different pitch; wherein thematerial comprises a plurality of first nanoparticles at a firstspatially varying density, wherein the first nanoparticles have asurface plasmon resonance at the first wavelength; and a plurality ofsecond nanoparticles at a second spatially varying density, wherein thesecond nanoparticles have a surface plasmon resonance at the secondwavelength.